1. Field of the Disclosure
The disclosure herein relates generally to correcting downhole measurements for clock drifts.
2. Description of the Related Art
To drill wells, such as hydrocarbon wells, a drill bit located at the end of a drillstring is rotated to drill into the formation. The rate of penetration (ROP) depends upon the weight on bit (WOB), the rotary speed of the drill bit, the type of the formation and the condition of the drill bit. The drill bit is attached to the bottom end of bottomhole assembly (BHA) that includes several formation evaluation (FE) tools or sensors that make measurements of formation properties. These tools include, for example, tools for determining density, porosity, resistivity, acoustic, nuclear and other properties of the formation surrounding the borehole during drilling of the wellbore. Similar measurements may also be made after the well has been drilled by conveying logging instruments on a wireline or coiled tubing. The BHA also includes other sensors for taking measurements relating to certain drilling and drillstring parameters, such as pressure, temperature, vibration, whirl, BHA inclination, weight-on-bit, rotational speed of the drill bit (RPM), etc.
These tools and sensors are longitudinally spaced (uphole of the drill bit). The distance of a tool or sensor from the drill bit is referred to herein as “offset” or “distance offset.” The offset for certain of these tools and sensors can be from several feet up to over one hundred feet. The determination of the properties of the formation and the drilling environment is based upon the evaluation of a suite of measurements taken along the borehole (logs) that are properly aligned in depth.
For proper depth alignment of logs, typically, all measurement data from MWD tools are referenced to the time at which such measurements are made. For MWD measurements, the time when each measurement is made or taken is presumed to be known. For applications involving correlation of a log or different logs, it is useful to determine where the measurement is taken, i.e. the wellbore depth at which each measurement is taken. To determine the depth at which each measurement is taken, it is necessary to know the time-depth profile, i.e. where (with respect to the wellbore) the drill bit is located at the time of taking each measurement. It is also useful to know the wellbore profile, i.e. the wellbore location in space. The wellbore profile often is determined using suitable survey instruments such as accelerometers and/or gyroscopes. Using the time-depth profile and the wellbore profile, estimates may be made to place the MWD measurement along the wellbore and hence in space.
The wellbore profile and time-depth profile are generally known only with some finite accuracy. This affects the accuracy of the final logs. The downhole measurements are time-stamped using a downhole clock associated with a downhole tool. Often, different tools include their own time clocks, while the surface measurements are time-stamped using a surface clock, often a computer clock. Downhole clocks are used in relatively harsh environments (high temperature, pressure and vibration) and often drift over time.
When the tool or downhole clock does not produce the same time as the surface clock during the entire drilling run, any attempt to use time-depth profiles (which are based on the surface clock time) to convert from when to where can result in erroneous depth placement of the measurement. The error depends on, among other things, the amount of the drift (mismatch) of the downhole clock from the surface clock. Due to the non-linearity of the time-depth transformation, even a relatively small time drift can result in noticeable errors or in unacceptable logs.
In a drilling environment, typically surface and downhole clocks are synchronized before the drilling run. The clock drift is often determined after the run from the difference in time between surface and downhole clock after retrieving the BHA. Such methods may produce less than acceptable correction results as the clock drift may be nonlinear over the drilling run. Accordingly, it is desirable to provide an alternative system and method for correcting downhole measurements due to clock drifts.